A typical metro-area network aggregates access traffic over a ring roughly 40-50 km in circumference, with typically 4-5 nodes interspersed along it. Each node serves as a portal for a local community through an access network, with residential and business users being added over time. Broadband access networks based on so called fiber to the home (FTTH), fiber to the curb (FTTC), or fiber to the building (FTTB) approaches are becoming the preferred technology for green field deployments. Passive optical networks (PONs) are increasingly employed for delivering broad-band wavelength based services from the metro-area network to the end users using optical fibers.
With reference to FIG. 1, a typical passive optical network consists of an optical line terminal (OLT) 10 performing optical-electrical-optical (OEO) conversion, a remote node (RN) 15 and a number of Optical Network Units (ONUs) 25 near end users. The OLT 10 can be located at a service provider's central office, where traffic coming from a metro edge node 5 in the form of a dedicated optical channel is received, undergoes the OEO conversions and is re-transmitted to the users. Typically, up to 32 ONUs can be connected to an OLT. The term “passive” indicates that optical transmission lines between the OLT 10 and the ONUs 25 have no power requirements or active electronic parts.
FIG. 2 illustrates one prior-art PON implementation, which is also known as the TDM-PON (Time Division Multiplexed Passive Optical Network). In this implementation, each ONU receives the same optical signal, which is generated by the OLT and broadcast to the ONUs 25 using a passive optical splitter 20 at the remote node 15. Each ONU may be assigned a time slot by the OLT 10 for receiving information designated for this ONU; data encryption can be used to prevent eavesdropping. Upstream optical signals are generated by the ONUs using a time division multiple access (TDMA) protocol, wherein the OLT provides time slot assignments for the upstream communication, and are combined at the remote node 15 in a single upstream optical channel, wherein signals from individual ONUs are time-multiplexed, which is then passively transmitted to the OLT 10. The upstream optical signals are typically at a different wavelength from the downstream signals. The OLT 10 typically communicates with the metro ring network 2 through an add/drop port of an optical add-drop multiplexer (OADM) located at the metro edge node 5 using a different set of wavelengths that is used for the OLT-ONU communications. The TDM-PON architecture combines the high capacity offered by optical fiber with the low cost of a passive infrastructure. The sharing of bandwidth in TDM-PON is limited to a single wavelength.
FIG. 3 illustrates another prior art PON implementation known as WDM-PON (Wavelength Division Multiplexed Passive Optical Networks), wherein the passive optical power splitter 20 in the remote node 15 is replaced with a wavelength de-multiplexer (DMUX) 21 to spectrally separate the wavelengths coming from the OLT 10 on the same fiber 13. In this implementation, the OLT has several transmitters and generates a dedicated optical signal at a different wavelength for each ONU 25, which provides an additional communication bandwidth.
FIG. 4 shows a hybrid PON architecture called TDM/WDM PON that is attractive because it provides more bandwidth than TDM PON by using several wavelengths, and allows more flexibility in sharing bandwidth amongst neighboring ONUs than WDM PON through TDM. The hybrid TDM/WDM PON architecture features many transmitters at the OLT10 and a remote node with a DMUX 21 followed by a passive optical power splitter 20 for each or some of the wavelength.
One drawback of the aforedescribed prior-art PON architectures is that they require OEO conversion at each OLT, wherein an OLT receives an optical signal coming from the metro ring network 2, and re-transmits as downstream traffic using one or several dedicated fixed optical transmitters; disadvantageously, the OLT represents a significant part of the cost of current PON architectures.
Another drawback of the prior art access network architectures in that the metro edge node 5, which serves as an interface between the metro ring network and the PON, typically utilizes a fixed optical add-drop multiplexer (OADM), so that wavelengths that are added and dropped at each metro edge node are fixed. This disadvantageously limits the flexibility of wavelength assignment in the metro ring network and possibilities for wavelength reconfiguration and re-use.
An object of the present invention is to provide a unified metro-access optical network architecture that would be free of at least some of these and other drawbacks of the prior art optical networks, and enable wavelength re-configurability and re-use at the metro-PON interface and in the metro-area network at a lower cost.